The present invention relates generally to the field of electronic test systems. More specifically, the invention provides an improved method and apparatus for performing probe tests on semiconductor wafers.
Various types of wafer test and probe equipment are well known to those of skill in the art, and are widely used in semiconductor manufacturing operations. Such equipment is used to provide electrical signals to a plurality of dies, generally formed on a semiconductor wafer, and to monitor the response of the dies to the electrical signals. Wafer test and probe equipment is made by a variety of manufacturers including, for example, Electroglas, KLA, Teradyne.RTM., Schlumberger, and Trillium.
Semiconductor wafer testing is normally conducted prior to wafer dicing and chip packaging. The wafer is placed on a prober chuck, indexed, and each die is tested. The testing operation normally involves placing a probe card with a number of probe tips in contact with a particular die at selected locations, such as, for example, the bond pads. Predetermined voltage patterns are then applied to the die, and the response of the die to the signals is monitored. If the die exhibits an appropriate response, the die is presumed to be "good." If the monitored response falls outside performance parameters, the die is either rejected, or appropriate remedial action is taken. Such wafer tests are performed on a wide variety of semiconductor products ranging from DRAMs and SRAMs to microprocessors.
Because modem semiconductor devices are being developed to operate at higher and higher speeds, "overhead" test techniques have been developed to keep signal transmission lines as short as possible. Short transmission lines reduce cross-talk between adjacent lines and eliminate other undesirable high frequency effects. In previous overhead testing techniques, a section of the tester referred to as the test head is positioned over the wafer under test and then docked. A printed circuit board comprising a plurality of downwardly extending pins is coupled to the test head which, when docked, causes the pins to come into contact with the wafer.
While enjoying some measure of success, such prior systems have also encountered certain difficulties. One such difficulty is related to the fact that the arrangement of pins and circuitry on probe cards varies from application to application. For example, the arrangement of pins on a probe card for testing 1M DRAMs is radically different from the arrangement of pins on a probe card intended for testing of 4M DRAMs. The arrangement of pins on a DRAM probe card differs even more dramatically from the arrangement of pins on a microprocessor probe card. Therefore, when the user of a prior system desires to switch the wafer product being tested, it becomes necessary to remove and replace the currently installed probe card. This is a time consuming and difficult task for a number of reasons. First, in many systems the probe card is mounted under the test head, which must be undocked before the probe card may be removed. This proves difficult because most test heads weigh between 250 and 750 pounds. Additionally, in most systems, the interface between the test head and the probe card must be disassembled and then reassembled. Finally, recalibration of probe card and test head positioning is required after replacement of a probe card.
Under some circumstances, it may be desirable to mount additional devices, such as transistors, resistors, and capacitors on a probe card. This highlights another problem encountered by prior systems. Because the vertical clearance between the probe card and the prober elements is often extremely constrained, the user's ability to mount such devices on a probe card is limited.
As device density increases due to advances in semiconductor fabrication, probe card pin density also increases. Thus, the performance of the probe card interface system has become increasingly critical to successful device testing. In order to ensure proper contact between probe card spring contact pins (i.e., Pogo.RTM. pins) and the probe card test, a significant force between the two must be maintained. For example, in some systems, probe card pins are commonly subjected to a force of about 500 pounds or more. Because of the often delicate nature of probe card pins, numerous touchdowns may result in probe pin "drift". Thus, probe cards often require maintenance after repeated contacts with semiconductor wafers.
Conventionally, those of skill in the art have had no effective way of recording the operating history of a probe card. Therefore, contact failures only have been detected after the fact using, for example, a method such as the one disclosed by U.S. Pat. No. 5,019,771, the entire specification of which is herein incorporated by reference. Using such reactive methods, however, a probe card is replaced and/or repaired only after a failure has occurred. This results in undesirable system down time.
The problem of maintaining records on individual probe cards is made difficult by the fact that probe cards are frequently moved in and out of a testing system as described above. One solution involves manually labelling the various probe cards, and manually entering information regarding probe card usage on data sheets or into a computer. Unfortunately, this becomes difficult or impractical when large numbers of probe cards are in use. Additionally, the procedure is highly susceptible to human error.
Two systems which present specific solutions to the above-described problems are disclosed in commonly assigned U.S. Pat. No. 5,254,939 for PROBE CARD SYSTEM AND METHOD, and commonly assigned, copending U.S. patent application Ser. No. 08/089,874 for PROBE CARD SYSTEM AND METHOD, filed on Jul. 9, 1993 as a continuation of the '939 patent, the entire specifications of which are herein incorporated by reference. The systems described in the '939 patent and the pending continuation application each comprise a plurality of cassettes into which individual probe cards are placed and positioned with stepper motors in a prober-test head interface. However, because of specific industry needs, other solutions are needed.
In view of the foregoing, an improved system and method for conducting wafer probe tests is needed which provides a sturdy and reliable interface to load probe cards into a test head. An improved system and method for tracking individual probe card usage and performance is also desirable.